In D.C. electric arc welders, the output circuit normally includes a capacitor in parallel across the electrode and workpiece with a relatively small inductance for charging the capacitor as the rectifier or power supply provides D.C. current. This inductance removes the ripple from the welding current. In series with the arc gap of the welder there is provided a large choke capable of handling high currents over about 50 amperes and used to control current flow for stabilizing the arc. As the feeding speed of the electrode toward the workpiece and the length of the arc change, the welding current varies. In the past, the large output choke in series with the arc had a fixed air gap in the core to control the inductance at a fixed value as current changes. However, when the choke experienced high weld currents, the core saturated and reduced the inductance drastically. For this reason, the width of the air gap in the core was enlarged to provide constant inductance over the operating current range of the welder. The choke was selected for a particular operating current range. However, this range would vary for different welding operations. Thus, the air gap of the choke was selected for the majority of welding operations. In a standard choke, a small air gap provided high inductance, but would saturate at relatively low currents. To increase the current capacity of the choke, the air gap was enlarged to reduce the amount of inductance for a particular size of the choke. For these reasons, the choke was made quite large with large wires to carry the weld current and a large cross sectioned core to prevent saturation. The gap was large to accommodate a wide range of welding currents. Such chokes were expensive and drastically increased the weight of the welder. Further, the choke produced a constant inductance until the saturation point or knee, even though ideal arc welding is realized with an inductance that is inversely proportional to the weld current. To alleviate these problems, it has been suggested that the air gap could include two or three different widths. This suggestion produced a high inductance until the small air gap saturated. Thereafter, a lower inductance would be realized until the larger air gap saturated. By using this concept of two, or possibly three, stepped air gaps, the size of the choke could be reduced and the range of current controlled by the choke could be increased. Further, the relationship of current to inductance was inverse. The concept of using a stepped air gap in the core of the output choke allowed a smaller choke; however, one or more inflection points existed. When the feed speed of the electrode or arc length changed to operate in the area of the inflection points, the D.C. welder would oscillate about the saturation or inflection points causing unstable operation. A standard swinging choke was not the solution because the weld current varied too much to operate on the saturation knee. In addition, such swinging chokes were for small current applications.
The use of a fixed output choke for a D.C. arc welder is now standard. Such choke is large and the operating point is in the linear portion of the inductance preventing drastic reductions in the output inductance of the welder. Such choke is expensive and heavy. By the procedure of having a stepped air gap, the size of the choke could be reduced and the current operating range increased; however, the inflection point at the saturation of one gap, made the welder less robust and susceptible to oscillation at certain arc lengths and feed speeds. Consequently, this suggested modification was not commercially acceptable.